stratocumulus clouds will form in the area. See figure.5-13.Saturation of the Air MassThe saturation curve in figure 5-14 shows theamount of moisture in grams per kilogram the air willhold at various temperatures.The air along the curve is saturated and is at itsdewpoint. Any further cooling will yield water as aresult of condensation; hence, fog or low ceiling clouds(depending upon the wind velocity) will form.Nocturnal CoolingNocturnal cooling begins after the temperaturereaches its maximum during the day. Cooling willcontinue until sunrise, or shortly thereafter. Thiscooling affects only the lower limits of the atmosphere.If nocturnal cooling reduces the temperature to a valuenear the dewpoint, fog or low clouds will develop. Thewind velocity and terrain roughness will control thedepth of the cooled air. Calm winds will allow a patchytype of ground fog or a shallow, continuous ground fogto form. Winds of 5 to 10 knots will usually allow thefog to thicken vertically. Winds greater than 10 knotswill usually cause low stratus or stratocumulus to form.See figures 5-15 and 5-16 for examples of fog andstratus formation.The amount of cooling at night is dependent on soilcomposition, vegetation, cloud cover, ceiling, and otherfactors. Cloud cover based below 10,000 feet has agreenhouse effect on surface temperatures, absorbingsome terrestrial radiation and reradiating a portion ofthis heat energy back to be absorbed by the land. Thiscauses a reduction in nocturnal cooling. Nocturnalcooling between 1530 local and sunrise will vary fromas little as 5° to 10° (with an overcast sky conditionbased around 1,000 feet), to 25° or 30°F with a clear skyor a cloud layer above 10,000 feet. Other factors andexceptions must also be considered. If a front isexpected to pass the station during the night, or onshorewinds are expected to occur during the night, the amountof cooling expected would have to be modified in lightof these developments.Figure 5-13.-How wind velocity can cause a low cloud layer.5-20